Talk:Sea salt aerosols

Incorrect growth factors in GET_ALK

The hygroscopic growth factors in GET_ALK (seasalt_mod.f) are incorrect.

The following:

        ! hygroscopic growth factor for sea-salt from Chin et al. (2002)
        IF ( IRH < 100 ) HGF = 2.2d0
        IF ( IRH < 99  ) HGF = 1.9d0
        IF ( IRH < 95  ) HGF = 1.8d0
        IF ( IRH < 90  ) HGF = 1.6d0
        IF ( IRH < 80  ) HGF = 1.5d0
        IF ( IRH < 70  ) HGF = 1.4d0
        IF ( IRH < 50  ) HGF = 1.0d0

should be replaced with:

        ! hygroscopic growth factor for sea-salt from Chin et al. (2002)
        IF ( IRH < 100 ) HGF = 4.8d0
        IF ( IRH < 99  ) HGF = 2.9d0
        IF ( IRH < 95  ) HGF = 2.4d0
        IF ( IRH < 90  ) HGF = 2.0d0
        IF ( IRH < 80  ) HGF = 1.8d0
        IF ( IRH < 70  ) HGF = 1.6d0
        IF ( IRH < 50  ) HGF = 1.0d0

Comparing modeled sea salt to observations

Xinyi Dong wrote:

I have another question about the species "sea-salt aerosol" in GEOS-Chem: according to the species list, there are aerosols SO4 (sulfate), SO4s (sulfate on surface of sea-salt aerosol), NIT, NITs, SALA, and SALC. So if I want the mass concentration of sea-salt, which of the following is right:

   sea-salt aerosol = SALA + SALC + SO4s + NITs, or
   sea-salt aerosol = SALA + SALC

And if I want the mass concentration of sulfate, which of the following is right:

   sulfate = SO4, or
   sulfate = SO4 + SO4s

Also, when GEOS-Chem is evaluated against surface observations (for example, IMPROVE), how do you handle the data from different observation sites that lie in the same grid-cell?

Jeff Pierce wrote:

I personally use this formula for total sea salt:

   sea-salt aerosol = SALA + SALC

If you want the real total sulfate, it's

   sulfate = SO4 + SO4s.

However, in most studies, when people say sulfate, they mean "non-sea-salt sulfate" in which case

  sulfate  = SO4 

is correct. I believe that SO4s and NITs are part of SALA and SALC (thus, if you wanted the sea-salt without the sulfate and nitrate that is associated with it, you subtract it). I'm not positive if this is correct though.

As to your question about different observations that fall into the same grid box: as fas as I know, different people do things differently. You can

1. evaluate you grid box against each of the observations (thus 3 obs sites would be 3 points on a 1:1 plot) or
2. average the various sites to have just a single comparison.

I generally prefer (1) since it highlights the variability within the gridboxes. Occationally, depending on what I'm evaluating, I'll remove observation sites that are in hotspots like cities (though if you are running at nested resolution, you resolve these better anyways).

Colette Heald wrote:

I concur with Jeff. My understanding is that SO4s is sulfate on sea salt, but it is simply the mass of sulfate on this medium (not the mass of sulfate+ sea salt). The same for NITs. So

   sea salt aerosol = SALA + SALC 

and

   total sulfate = SO4 + SO4s

Jeff is also right that there are different ways to compare model and obs. I actually prefer his method #2 (although I calculate the standard deviation of observation sites that I average within a gridbox to track the idea of variability that Jeff mentioned).

--Bob Y. 12:15, 25 February 2013 (EST)